Machines; Alternators Flashcards
Another name for alternators?
Synchronous generators or alternating current generators
Where is voltage induced in an alternator?
Armature
Induced voltage generated by the alternator is (AC/DC)?
Ac
What does the exciting current create?
The magnetic field
What kind of source supplies the exciting current?
An external DC source
What kind of loads are appropriate for a rotating Armature alternator?
Lower kVA capacities
What is the most common type of alternator?
1 with rotating field windings
What is the Armature connected to in a rotating field alternator?
Directly connected to an external load
What is the rotor connected to in a rotating field alternator?
Connected to a DC Source through slip rings and brushes (to create rotating magnetic field)
What are some advantages of rotating field alternators?
1) Slip rings and brushes not required to connect the Armature output to the external load
2) field requires relatively low currents and voltages (smaller brushes and slip rings)
3) structure can be physically smaller
4) Armature windings easier to cool
What are the two distinct types of synchronous generator rotating field structures?
Cylindrical rotors and Salient pole rotors
How many magnetic poles would we expect to find in a cylindrical rotor?
2 or 4
What is the practical application of cylindrical rotors?
Alternators that require to be driven at high speeds (high speed prime movers: gas or steam turbines)
What’s speeds would we expect a cylindrical rotor to be driven at?
1800 or 3600 r/min
What kind of prime movers might drive a Salient pole rotor?
Internal combustion engines water turbines or electric motors
Where does the mechanical to electrical energy conversion happen in an alternator?
In the Armature winding
What is the relationship between number of windings in a stator and and voltage generated?
Proportional
How many degrees apart are the windings in a 3-phase alternator?
120 mechanical degrees
How many degrees apart are the voltages produced by the windings of a three-phase alternator?
120 electrical degrees
What are mechanical losses?
- bearng friction
- friction between the brushes and slip rings
- windage friction of the rotor
- and the power required to drive the built-in cooling fan
What supplies energy for mechanical losses in a alternator?
Prime mover
Which losses change width the load and which do not?
Iron losses do not change with the load
Copper losses do change with the load
What kind of losses are Eddy currents and hysteresis?
Iron losses or core losses
What do we use to reduce the effects of Eddy currents?
Laminations
What are Eddy currents?
Magnetic flux induces voltage within the iron which causes currents, causing heat loss in the core.
What do we use to reduce hysteresis losses?
Use Alloys in the core materials
What causes hysteresis losses
The magnetic retentivity of the core
What are copper losses?
Heat losses produced by current in the windings of the alternator
What do we do to reduce copper losses?
Use coolants such as hydrogen gas or water to reduce the temperature of the Armature conductors
How much do Armature losses decrease when the load to an alternator is decreased by a factor of 2?
Armature lost is decreased by a factor of 4
Do rotational losses remain fixed or change with the load
Remain fixed because of the constant rotational speed
What happens to the overall efficiency of an alternator from full load to half load condition?
Efficiency decreases from a full load to a half load condition
What does the value of voltage that is generated in the Armature of a rotating field alternator depend on?
1 the number of Armature turns per winding
2 the speed of the rotating field
3 the amount of flux produced by the rotating field ***
What is the only practical way to change the value of voltage that is generated an Armature of a rotating field alternator?
Amount of flux produced by the field
How can we change the amount of flux produced by the field?
1 Number of turns in the field windings
2 type of core material
3 amount of current flowing through the feild windings
What is the only factor that can be altered after a machine has been constructed ( in order to change the value of voltage that is generated)?
Field excitation current
True or false an alternators output voltage (terminal voltage) is affected by the load it supplies
True.
When is generated voltage on the Armature windings equal to the terminal voltage?
When the alternator is not loaded and no current is flowing
Why does the terminal voltage differ from the generated voltage as load is added to an alternator?
Because Armature resistance
Armature reactance and
armature reaction
produce voltage drops.
What is synchronous reactance made up of?
Inductive reactance and armature reaction
Which voltage drops are proportional to Armature current?
Inductive reactance and armature reaction voltage drops
What is synchronous impedance made up of?
Resistance of the Armature and synchronous reactance
What can be use to calculate the maximum short circuit current available from the output terminals under fault conditions?
Synchronous impedance
Why is it important to know the maximum short circuit current?
In order to size protective equipment
What would we expect from a percent voltage regulation calculation if the full load voltage is higher than the no load voltage?
Percent voltage regulation would be negative
True or false. V(rarm) and V(xsych) are affected by the power factor of the load
False. They are independent of the power factor of the load.
True or false generated voltage is affected by the power factor of the load
True
What sources of excitation are used for separately excited AC alternators?
Battery Banks
DC generators
solid-state rectifiers
What type of excitation source uses a relatively small shunt or compound DC generator mounted on the same alternator shaft that supports the main alternator rotor and field windings?
Brush type self-excited alternator
Which type of excitation system does not require brushes slip Rings or a commutator?
Brushless excitation system
Which type of excitation system uses a small alternator and a full wave Bridge rectifier?
Brushless Exciter
How does a voltage regulator work
Reducing resistance with a rheostat increases current flow to the rotating field (via the brushes and slip rings)
Increased current generates more flux and raises the alternator ac voltage back to its proper level
What is an automatic voltage regulator?
Solid-state device that automatically regulates voltage
What four conditions must be met before an incoming alternator is in synchronization with a running alternator?
Following must be the same: 1 phase sequence 2 terminal voltages 3 frequencies 4 voltage phase position
What is the most common method used to check phase sequence?
Three dark lamp method
What is Phase sequence?
The direction of the magnetic field rotation
What is indicated when the three lamps remain dark?
Phase voltages from each alternator have the same phase sequence frequency and phase position.
What value of voltage must the lamps (of a three-dark-lamo set up) be suitable for?
Twice the alternators phase voltage.
What does a clockwise rotation on a synchroscope indicate with regards to paralleling alternators?
Frequency of incoming alternator is higher than if the running alternator or supply
List three advantages of connecting alternators in parallel
1 increase output capacity of a system
2 can disconnect paralleled alternator for maintenance and repair without power interruption
3 can allow for more efficient operation of the alternators over a wide range of loads
What is indicated by three lamps diming and brightening at the same time?
Same phase sequence Different frequency (lamps flash at a rate equal to the difference in frequency)
What is indicated by three lamps flashing in sequence?
Different phase sequence
Different frequency
What is indicated by three lamps remaining bright all the time
180° phase displacement
Same phase sequence
Same frequency
What does it mean when an alternator is floating
Been synchronized with another alternator but not yet supplying any load
When transferring powerload what would happen if you increase the power input to alternator to without decreasing the power input to alternator one?
The total power input increases resulting in an unwanted increase in speed frequency and generated voltage of both alternators
How do you transfer power load without changing frequency?
Increase the power input or the torque of the prime mover of the incoming alternator while simultaneously decreasing on the running alternator
What sort of power factor is required to make a reactive power load transfer?
A lagging power factor
How do you increase the power load on a paralleled alternator?
Increase the torque of the prime mover
How do you increase the amount of reactive power load delivered by a paralleled alternator?
Increase field excitation
Why would we want to operate each parallel alternator at the same power factor?
To reduce the circulating current
How do you adjust the output voltage of parallel alternators?
Proportionally adjusting the field excitation of both alternators (increase both at same rate)
How would you raise the frequency of a parallelled alternator system?
Increase the power input to both alternators by increasing the speed of their Prime Movers
What would you adjust to ensure an incoming alternator supplies its proper share of the load?
Adjust the prime movers
What would you adjust to ensure each alternator delivers its share of current?
Field excitation
How would you raise frequency of a paralleled alternator system?
Increase speed of both prime movers (to increase power input)
How would you adjust the output voltage of a paralleled alternator system?
Adjust field excitation of both alternators in the same direction.
What would be the result of increasing the field excitation of one alternator in a paralleled system?
Power factor of that alternator would shift in a lagging direction. Current output would increase on both.
What happens to frequency within a paralleled alternator system if load decreases? How could this be fixed?
Frequency increases. Adjust power input of the prime movers
When does “hunting” occur?
When the phase position of two paralleled alternators are not the same
What generates synchronizing power?
Circulating current and voltages of each alternator.
Which alternator produces synchronizing power and which alternator consumes it?
Produced by leading alternator
Consumed by lagging alternator
What can reduce hunting?
Heavy flywheel
Embedded amortisseur windings (damper windings)
What do damper windings do?
Produce an induced current that opposes and limits the action of hunting
